1. Field of the Invention
The invention is in the field of time measuring exposure and/or controlling circuits for the time of exposure of a photograhic camera. A measuring device is utilized for measuring the brightness of the object and emitting an exposure time signal after a time interval has elapsed which corresponds to the correct exposure time.
2. Description of the Prior Art
In known exposure measuring and/or control circuits of this type an oscillator is actuated during the initiation of a measuring phase by depressing the measuring key of the camera. The measuring device comprises a silicon photodiode connected in parallel to an integrating condenser. The parallel connection of the photodiode and the integrating condenser is inserted in a parallel circuit with gate-drain terminals of an FET. The source of the FET is connected on one side with a comparator and on the other, by means of an RC element, with the output of the oscillator. The output of the oscillator is further connected with an indicating device in the form of a light emitting diode (LED). This LED is further connected with a translator actuated by the comparator of the measuring device.
Upon the closing of the measuring key and the actuation of the oscillator, pulses of constant frequency appear at the output of the oscillator. These pulses are differentiated by way of the RC element and because of the diode effect between the gate and the source of the FET, a current opposed in direction to the photocurrent will flow whenever the negative-going voltage pulse appears from the output of the oscillator, in the integrating condenser of the measuring device. An equilibrium is established between the discharge of the integrating condenser by the photocurrent and the pulse current, and a certain voltage potential will appear at the source of the FET. When the voltage potential is less than the reference voltage of the comparator, the transistor remains blocked, and the LED will blink in accordance with the oscillator frequency. When the voltage potential approaches the reference voltage with increasing brightness, the LED will blink with a rising duty cycle and will emit light longer with a constant blinking period. If the luminous brightness of the object is sufficient for an adequate exposure, or if it is greater than necessary, a voltage potential that is higher than the reference potential of the comparator will appear at the source of the FET during the second half of the oscillator period. This will render the transistor conducting at the outlet of the comparator and the LED will emit light constantly. The reference potential of the comparator is set so that the threshold at which the comparator will actuate the transistor corresponds to an exposure time of 1/30 s. The blinking of the LED indicates to the photographer that the exposure time to be expected is greater than 1/30 s and that therefore the danger of jumping exists during manual exposures. The constant glowing of the LED indicates that an exposure time shorter than 1/30 s is to be expected. If the exposure time to be expected is slightly longer than 1/30 s, the LED will emit light over increasingly longer periods and approaches constant emission from the point of view of the photographer.
With this known circuit layout, the indicating device is able to signal a single threshold only. In the final analysis, the user of the camera knows only whether the exposure time to be expected upon triggering the camera will be shorter or longer than 1/30 s. In addition, the 1/30 s threshold is indicated not in a definite manner, but possesses a certain uncertainty or bandwidth, because as a consequence of the increasing time of actuation of the indicating device within a blinking period in the vicinity of the threshold the user of the camera is no longer able to distinguish the very slight blinking and the constant emission of the indicating device.